Bell solves BA609 tiltrotor torque mystery

Flightglobal.com ^ | 06/10/07 | John Croft

[Freeport]

Bell Helicopter engineers have resolved a performance anomaly that test pilots had discovered when flying the hybrid aircraft in conventional flight mode, also known as the zero-degree position of the nacelles. The BA609 is a six- to nine-passenger corporate aviation tiltrotor, a product the companies expect to begin delivering in 2011.

According to Roy Hopkins, Bell's chief test pilot for the BA609, both prototype aircraft had been exhibiting a differential torque between the two prop-rotors in flight tests, causing a left-turning tendency at cruise speeds. The companies are flight testing one aircraft at Bell in the USA and the other at partner AgustaWestland's facility in Italy.

After being briefed on the quirk, Hopkins says Bell engineers homed in on the most unlikely of culprits - the placement of the exhaust port on the left engine. The aircraft's Pratt & Whitney Canada PT6C-67A turboshaft engines have a single exhaust port on the outboard side of the nacelle. The BA609's sibling, the Bell Boeing V-22 Osprey, has downward facing exhaust ports for its Rolls-Royce engines, and has not experienced a similar issue.

Further analysis and testing proved that turbine exhaust exiting the exhaust stack of the left side PT6C-67A turboshaft was swirling into the prop wash in a way that was causing extra drag on the left side. The same was not true on the right side of the aircraft where, due to the layout and direction of prop rotation, the exhaust flows into the slipstream.

The fix - extending the exhaust port farther down along the left nacelle - was put in place two weeks ago and tests were to resume in earlier this month. If cruise tests indeed prove the problem to be fixed, the company will install the correction into the Italian prototype, says Hopkins.

[Boundless]

> Doesn’t Rotax manufacture engines?
> I don’t get the flying on a tether part.

You are correct. I was thinking of Moller.
http://www.moller.com/
which presently has trademarks on “Rotapower” and “RotaFan”

[Boundless]

> Now, for all the naysayers who would prefer
> America should never take a risk ...

I’ve no problem with risk, as long as it’s plainly stated.

Tilt-rotors are much riskier than fixed wing, and probably
riskier than traditional rotary wing.

And my statement about cert wasn’t an endorsement of the
present FAA posture. It’s just a political reality that
novel flying machines have to face, and they can’t assume
they’ll get a special type rating. I’m encouraged that
the FAA isn’t very much standing in the way of commercial
suborbital vehicles, which are clearly extremely risky.

[A.A. Cunningham]

Can this thing auto-rotate like a helo?
Guess: barely, and only if the blades are horizontal.

Helicopters can attempt to autorotate, there's no guarantee that said attempt will be successful; most aren't. Any idea what the disc loading on the 609 is and don't guess?

Can it glide like a fixed wing?
Guess: barely, and the touchdown speed will be dangerously high.

Any idea what the glide ratio on the 609 is and once again, don't guess?

[aruanan]

After being briefed on the quirk, Hopkins says Bell engineers homed in on the most unlikely of culprits - the placement of the exhaust port on the left engine.

I bet Richard Feynman would have spotted this immediately.

[G Larry]

You should read some of the V-22 blogs first.